Surgical sutures made from polypropylene have been successfully used by the medical profession for more than twenty years. One of the first commercially successful polypropylene suture to gain wide acceptance was described in U.S. Pat. No. 3,630,205 to Gregory J. Listner. Listner disclosed a process for manufacturing polypropylene suture that comprised the steps of drawing an extruded polypropylene suture to about 6.6 times its original extruded length and then relaxing or shrinking the monofilament to between about 91 to 76 percent of the stretched length.
Lennard et al. in U.S. Pat. No. 4,911,165 later disclosed an improved process for making polypropylene sutures that have increased compliance, limpness or flexibility to make the polypropylene sutures easier to handle and improved their knot security. The process described by Lennard comprised extruding polypropylene through an orifice and quenching the extrudate to form a filament. The filament was first drawn about 6.times. to 7.times. then drawn a second time from about 1.06.times. to about 1.5.times.. Finally the filament was collected and heat relaxed off-line to effect a linear shrink and heat set. The heat relaxation as disclosed by Lennard is performed by placing the filament on a rack in an annealing oven and allowing the filament to shrink from about 16 to about 35 percent of the original length of the filament. The annealing is carried out at a temperature within the range of from about 135.degree. C. to about 152.degree. C., for a period of time sufficient to permit the filament to shrink and heat set normally 5 to about 40 minutes.
As good as the fibers are that may be produced from the processes disclosed by Listner and Lennard there is room for improvement in these processes. In particular it would be desirable to eliminate the separate annealing step performed on the filaments after the filament has been draw to stream line the suture product and handling.
Thus it is an object of the present invention to provide a process for producing polypropylene suture that eliminates the rack annealing of polypropylene sutures. This and other objects and advantages of the present invention will be obvious to those skilled in the art from the following specification.